Dr. Kent Brantly, the American doctor who contracted Ebola while working with relief organization Samaritan's Purse in Liberia, told NBC in an exclusive interview that he felt like he was about to die when he was isolated in a Liberian hospital.

"I said to the nurse who was taking care of me, 'I'm sick. I have no reserve. And I don't know how long I can keep this up,'" the Fort Worth doctor told Matt Lauer in Asheville, North Carolina, where his family has been living in seclusion since he left an Atlanta hospital almost two weeks ago.

A portion of the interview, Brantly's first ever, was airing Tuesday on "Nightly News," and another portion will air Wednesday morning on the "Today" show. The full report will air in an hour-long NBC News primetime special on Friday.

"I said, 'I don't know how you're going to breathe for me when I quit breathing,'" Brantly recalled telling the nurse in Liberia. "'Cause that was the reality. I thought, 'I— I'm not gonna be able to continue breathing this way.' And they had no way to breathe for me if I had to quit breathing."

Brantly: "I Am Thrilled to Be Alive"

Brantly's wife Amber, who lived with her husband and children in Liberia, and the doctors who treated him at Emory University Hospital have also spoken with Lauer, and NBC News cameras had exclusive access to the isolation room where Brantly was treated.

Amber described to Lauer how scared she had felt when she found out her husband had contracted Ebola.

"I knew what was coming," she said. "I knew how it ends. I knew how everyone had ended up so far."

Brantly was flown out of Liberia a month ago after contracting Ebola in July. He was isolated and spent three weeks being treated at Emory University Hospital before he was declared cured and discharged on what he called "a miraculous day."

Aid worker Nancy Writebol, 59, was also flown to Emory Hospital for treatment in early August and was quietly released several days before Brantly, also cured of the potentially deadly disease.

On Tuesday, NBC News reported another American missionary doctor has tested positive for Ebola, the aid group SIM USA said. The doctor was treating obstetric patients at ELWA hospital in Monrovia, Liberia, though not in the hospital’s isolation unit, NBC News reported.